Signs are well known for purposes such as conveying information or for aesthetics, or some combination of each, and are produced on a variety of media. Wooden signs are a particularly popular style, especially such signs that include raised letters, numbers or other graphic images. As used herein, the term "wood" is intended to include wood and other carvable products including but not limited to plastics, metals, and other suitable materials. The raised letters, numbers or other graphic images are defined by an initial graphic representation, which is commonly referred to by lay persons as a two-dimensional "outline," although those skilled in the art will recognize that the term "outline" has a different, distinct meaning in the art.
The raised letters, numbers or other graphic images, as viewed from a position in front of the sign, each have two beveled sides or "facets" located within the initial graphic representation. The facets abut along an edge to form an apex, typically along the center of the lettering. The letters, numbers or other graphic images may include a "return," which is a measure of the distance between the lowermost edge of the facets and adjacent background. Such letters, numbers and other graphic images are referred to generally as "prismatic lettering" or "convex lettering," since the letters, numbers or other graphic images have a generally prismatic, convex cross-section.
Wooden signs with raised lettering, including those with prismatic lettering, have been produced for many years by hand-carving the sign from a wooden block, thick wooden board, metal, plastic or other suitable material, referred to herein as "sheet material." Hand-carving a wooden sign requires an experienced craftsman, a number of different and expensive tools, and a large amount of time to produce a sign. For aesthetic reasons it is usually desirable that such a sign be produced from a single block or thick sheet of wood.
To produce a hand-carved sign, a craftsman selects a piece of sheet material, stencils a proposed design onto the sheet material, and removes sheet material from areas within the design to produce rough letters, numbers of graphic images on the sheet material. He or she then bevels the top surface of the rough letters to form the prismatic lettering.
It is difficult to repair the sign if too much material is removed. Since wooden blocks, thick sheets of wood and other suitable material are expensive, it is imperative to avoid mistakes which result in removing too much material. Accordingly, the craftsman must proceed in a deliberate manner, which adds to the labor-intensive nature of hand-carved signs. It can take several days to produce a nominally-sized hand- carved sign, e.g., 3 feet by 4 feet. In addition, it is difficult to have a hand carved sign made, as only a few experienced craftspersons remain. As a result, hand-carved signs are expensive, and are prohibitively expensive for most applications. Larger and/or more intricate signs require even more time to produce, and are correspondingly more expensive.
A proposed solution to hand-carving signs is to produce the signs using a computer-controlled cutting machine that moves a cutting tool in a tool path of three or more dimensions, i.e., in X, Y and Z coordinate directions in a single pass. A typical cutting tool has one or more cutting faces arranged about an axis of rotation to selectively remove material from the sheet material when lowered into the material and moved in a tool path. Machines that are capable of operating in a three dimensional tool path are more expensive than machines that operate in only two or two and a half dimensions, but are becoming increasingly more common. The term "two dimensions" refers to the fact that a machine is capable of moving an associated cutting tool in only a two dimensional tool path at any given time, and is manually moved in the third dimension, i.e., indexed, in between tool paths. The term "two and a half dimensions" refers to the fact that a machine is capable of moving an associated cutting tool in only a two dimensional tool path at any given time, and is capable of automatically indexing the tool in between tool paths. Known cutting machines operating in three or more dimensions generate incised lettering, rather than prismatic lettering.
Machines capable of operating in a five dimensional tool path, the X, Y and Z axes, together with a capability for rotation about the axis of rotation of the cutting tool in two, additional directions, are extremely expensive, and there are relatively few such machines available. In contrast, machines that operate in three or fewer dimensions are substantially less expensive. Moreover, the number of computations that must be made to generate the information to operate the cutting head in five dimensions, and the corresponding time and memory needed to accommodate such computations, in order to generate corresponding tool paths is very substantial.
The width of the facets, and thus the size of the prismatic lettering that may be produced in a single pass, is limited by the radius of the cutting tool. While the limitation is of little concern for relatively small prismatic lettering, such a limitation has prohibited the application of automating the production of larger signs having prismatic lettering. This limitation is not solved simply by using a larger cutting tool for at least two reasons.
First, for cost reasons, it is desirable to use a smaller cutting tool. The cost of fabricating a cutting tool increases significantly with an increasing cutting tool radius. Thus, if a good-quality cutting tool with a one inch radius costs about $100, a cutting tool having a two-inches radius can cost several hundreds of dollars.
Second, for safety reasons, it is also desirable to use a smaller cutting tool. The cutting tools operate at speeds of over 20,000 revolutions per minute (rpm). Given the high rotational speeds, any imperfections in the cutting tool, such as a slight lack of balance, produce vibration that will, eventually and unpredictably, result in catastrophic failure of the cutting tool. Catastrophic failure in turn can result in serious damage to the cutting machine, and any other equipment or persons around the machine. Those skilled in the art will recognize that cutting tools with larger radii are more prone to vibration than smaller tools, and will produce more spectacular catastrophic failures than will tools of smaller radii. Part of the increased cost of a cutting tool having a larger radius results from the precision machining necessary to produce the cutting tool relatively free from imperfections such as lack of balance.
It is accordingly an object of the present invention to provide a method of producing signs having prismatic lettering that overcomes the drawbacks and disadvantages of prior art methods for producing signs having prismatic lettering.